ARTICLE IN PRESS
Applied Radiation and Isotopes 68 (2010) 993–1005
Contents lists available at ScienceDirect
Applied Radiation and Isotopes
journal homepage: www.elsevier.com/locate/apradiso
Review
Compton scatter imaging: A tool for historical exploration
G. Harding a,, E. Harding b
a
GE Security Germany GmbH, Heselstuecken 3, D-22453 Hamburg, Germany
University of Muenster, Interdisciplinary Research Training Group ‘‘Societal symbolism in mediaeval and early modern times’’ (German Research Foundation, DFG), Pferdegasse 3/
D-48143 Münster, Germany
b
a r t i c l e in fo
abstract
Article history:
Received 22 July 2009
Received in revised form
21 January 2010
Accepted 21 January 2010
This review discusses the principles and technological realisation of a technique, termed Compton
scatter imaging (CSI), which is based on spatially resolved detection of Compton scattered X-rays. The
applicational focus of this review is to objects of historical interest. Following a historical survey of CSI,
a description is given of the major characteristics of Compton X-ray scatter. In particular back-scattered
X-rays allow massive objects to be imaged, which would otherwise be too absorbing for the
conventional transmission X-ray technique. The ComScan (an acronym for Compton scatter scanner) is
a commercially available backscatter imaging system, which is discussed here in some detail. ComScan
images from some artefacts of historical interest, namely a fresco, an Egyptian mummy and a mediaeval
clasp are presented and their use in historical analysis is indicated. The utility of scientific and technical
advance for not only exploring history, but also restoring it, is briefly discussed.
& 2010 Elsevier Ltd. All rights reserved.
Keywords:
Compton scatter
Radiology
History
Cultural application
Contents
1.
2.
3.
4.
Historical overview of X-ray scatter imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994
Summary of compton effect physics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994
2.1.
Energy of compton scatter photon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994
2.1.1.
Angular distribution of compton scatter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995
2.1.2.
Compton profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995
2.1.3.
Material characterization using the compton profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996
2.2.
Comparison of scatter and transmission imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996
2.2.1.
Backscatter measurement geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997
2.2.2.
Density sensitivity of transmission radiography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997
2.2.3.
Density sensitivity of Compton radiography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997
2.2.4.
Tomography with scattered and transmitted X-rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997
Compton X-ray scatter imaging techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998
3.1.
Compton imaging topologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998
3.1.1.
Point-by-point imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998
3.1.2.
Line-by-line imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998
3.1.3.
Plane-by-plane imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999
3.1.4.
Energy-coded scatter imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999
3.2.
Densitometry with scattered X-rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999
3.2.1.
Numerical corrections for attenuation effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999
3.2.2.
Dual energy determination of attenuation factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000
3.2.3.
Multiple scatter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000
3.2.4.
Multiple scatter reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000
3.2.5.
Multiple scatter calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000
3.2.6.
Multiple scatter modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
Description of ComScan system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
4.1.
General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
4.1.1.
Radiation source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
4.1.2.
Primary beam collimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
Corresponding author.
E-mail address: Geoffrey.Harding@ge.com (G. Harding).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apradiso.2010.01.035
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5.
6.
G. Harding, E. Harding / Applied Radiation and Isotopes 68 (2010) 993–1005
4.1.3.
Detector array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
4.1.4.
Scatter collimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002
4.2.
Data acquisition and processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002
Historical applications of ComScan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002
5.1.
Scientific approaches to history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002
5.2.
History and natural sciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002
5.3.
Applications and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004
1. Historical overview of X-ray scatter imaging
Optical visualisation of our surroundings is based on stimulation of
the retina by scattered light. Analogously, scattered X-rays may be
used to probe objects that are invisible to the naked eye. There are
several different forms that X-ray scatter can take (e.g. fluorescent,
coherent, inelastic, etc.) corresponding to the type of physical
interaction that is responsible for it. In this review we focus on
Compton scattering, so named after the American physicist, Arthur H.
Compton, who was awarded the Nobel Prize in 1927 for his
interpretation of the X-ray scatter effect that bears his name.
Although Compton scattering is important in its own right as
providing one of the first demonstrations of the dual nature—particle
and wave—of light, it has subsequently been employed as the basis
of an image-forming technique for materials that are not transparent
to visible light. Compton scatter imaging (often abbreviated to CSI)
was first described by Lale (1959) in the medical field and his work
inspired many publications on Compton scatter radiography. In
restricting this introduction to applications of Compton scatter
imaging outside of the medical field, it is in no way intended to
detract from his pioneering work. For further information on
Compton scatter imaging in medical diagnosis, reference can be
made to the review article of Guzzardi and Licitra (1988).
Historically, the next field to which CSI was applied was nondestructive testing (NDT); and this was mainly stimulated by three
attractive properties of the scatter technique. First it is possible to
place both the radiation source and the detector on the same side of
the medium when back-scattered radiation is recorded. This is clearly
advantageous when surface layers of voluminous structures (e.g. thick
walls, highways, bridges, etc.) are to be examined as it avoids the
necessity for a transmission measurement. CSI is indeed an excellent
imaging technique for the many structures of historical interest that
are only accessible from one side. Compton back-scatter has a long
tradition of use as a probe of density for bulk homogenous materials
including soil (Christensen, 1972), concrete (Adil, 1977) and other
materials (Gautam et al., 1983; Charbucinski and Mathew, 1982;
Arkhipov et al., 1976; Bukshpan et al., 1975). Such density probes
usually monitor the energy spectrum of back-scatter flux induced on
irradiation with a wide-angle beam of gamma-radiation. The flux and
in some cases the shape of the back-scatter spectrum give
information on the density and composition of the scatter medium.
Careful calibration of these devices using materials that closely
resemble those under investigation is essential. Because in most cases
a single detector views all the radiation scattered back out of the
object they are relatively insensitive to depth variations in material
composition. Nevertheless the application of Compton densitometry
to the detection of buried land-mines as originally discussed by Roder
and Van Konynenburg (1975) is still the subject of ongoing
investigation (Wehlburg et al., 1995).
A second attractive feature of Compton scatter is its greater
sensitivity than the transmission mode to density variations in
low-density materials, such as gases (Bayly, 1962). We will
consider in Section 2.2 in more detail the conditions under which
a scatter measurement is to be preferred to transmission from the
point of view of contrast sensitivity.
Finally the fact that Compton scattering allows direct 3-D
spatial definition with high contrast resolution as found by Lale
(1959) motivated the AIDECS system, an acronym for automated
inspection device for explosive charge in shells, (Stokes et al.,
1982) and the ComScan (Compton Scatter Scanner) device
(Harding et al., 1984). The ComScan will be considered in more
detail later in this review as it has found a wide range of
application and is the only commercially available tomographic
Compton scatter imaging device of which this author is aware.
This review presents some Compton scatter images of objects
of historical interest obtained with the ComScan device.
A discussion of X-ray scatter imaging cannot be termed
complete without at least a mention of the interesting fact that
coherent scatter is the dominant contribution to the low angle
scatter in medical or industrial radiography (Neitzel et al., 1985).
This recognition has prompted the development of X-ray diffraction imaging, which is increasingly used for explosives and drug
detection (Harding and Harding, 2007). A discussion of these
techniques is outside the scope of this review; hence the
interested reader is referred to the literature.
The next section summarizes the most important physical
relationships of the Compton effect over the photon energy range
from 10 keV to 10 MeV, which is of interest in X-ray scatter
imaging applications.
2. Summary of compton effect physics
The importance of Compton scattering relative to other interaction phenomena can be judged from Fig. 1, showing the various
components of the attenuation coefficient in aluminium for photon
energies between 10 keV and 100 MeV. Aluminium has been chosen
as roughly representative of the low and moderate atomic number
(Z) materials (e.g. wood, stone, etc.) commonly encountered in
archaeological Compton scatter imaging. Compton scattering
dominates the attenuation coefficient over the energy range,
important for radiographic imaging, from 100 keVrE0 r10 MeV
and only varies by a factor of 10 between these two energy limits.
2.1. Energy of Compton scatter photon
The Compton effect is described in more detail in review works
such as those of Cooper (1985) and Williams (1977); hence it is
only necessary to summarise the essential relationships here.
Compton scattering refers to the interaction of X-ray quanta with
those atomic electrons whose binding energies are much less than
the energy transferred on scattering. An interesting semi-classical
account of the scattering process (Bohm, 1966) assumes that an
electron is accelerated by the radiation pressure of the incoming
wave until its de Broglie wavelength lB ¼ h=me v (v =electron
velocity and me its relativistic mass) is equal to the wavelength of
the incoming photon.
The Doppler shift of the wave emitted by the moving electron
leads to Compton’s equation (Eq. (1)) in which E1 is the energy of
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100
2000
10
1800
total
0.1
0.01
coherent
pair
0.001
0.0001
0.01
Compton
photo
0.1
1
10
Photon energy in MeV
45°
1400
1200
1000
800
90°
600
100
400
200
Fig. 1. Contributions to total attenuation coefficient in AI from 10 keV to 100 MeV.
Values of partial interaction coefficients are from Berger and Hubbell (1987).
180°
0
0
2000
the secondary and E0 that of the primary radiation:
E1 ðyÞ ¼
995
1600
1
Scatter energy (keV)
Attenuation in cm2 g-1
G. Harding, E. Harding / Applied Radiation and Isotopes 68 (2010) 993–1005
E0 me c2
ðme c2 þ E0 ½1cos yÞ
ð1Þ
4000
6000
Primary energy (keV)
8000
10000
Fig. 2. Compton energy dependence (cf. Eq. (1)) on energy of primary radiation for
angles of scatter of 451, 901 and 1801.
y is the angle of scatter in Eq. (1) and mec2 is the rest mass energy
of the electron (511 keV). The energy E1 is plotted against E0 for
several values of scatter angle in Fig. 2. Eq. (1) shows that the
energy of 1801 scatter is mec2/2 when E0 b mec2.
0
340
350
330
320
2.1.1. Angular distribution of compton scatter
The angular distribution of Compton scatter from free,
stationary electrons is given by the Klein–Nishina cross-section
dsKN(y)/dO which, for unpolarized photons, is as follows:
" #
E2 E1 E0
ð2Þ
þ sin2 y
dsKN ðyÞ=dO ¼ r02 =2 12
E0 E0 E1
and is expressed in units of m2 sr 1 electron 1. In Eq. (2), r0 is the
classical electron radius (2.82 10 15 m) and y is the scatter
angle. This cross-section is shown in Fig. 3 for several photon
energies. At high energy it becomes increasingly peaked in the
forward scatter direction (y-01). At low energy E1 EE0 and the
angular distribution in Eq. (2) reduces to the Thomson form of
(1+ cos2 y), which is symmetrical about the angle y = 901.
10
1
20
30
40
0.8
310
50
0.6
300
60
0.4
290
70
280
0.2
80
270
0
90
260
100
250
110
240
120
230
130
220
2.1.2. Compton profile
Compton’s energy shift formula was derived assuming stationary, free electrons. If the electron is bound (by Coulomb
attraction to the nucleus) it is necessarily in motion and this leads
to an energy broadening of the Compton scattered line, in analogy
to the classical Doppler shift of the signal emitted by a moving
source. The core electrons of heavy nuclei move with relativistic
speeds: even the bound electron of the hydrogen atom moves at
2.2 10 6 m s 1, corresponding to the hydrogen ground state
momentum of 2 10 24 kg m s 1. The shift in frequency of the
scattered photon is proportional to the magnitude of the
component of electron momentum projected along the direction
of the scatter vector, as discussed in standard works (Williams,
1977).
The electron momentum, q, for which a photon of energy E0
results in a scattered photon of energy Es at the angle y is
q ¼ 137
½E0 Es E0 =me c2 Es ð1cos yÞ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
E20 þE2s 2 E0 Es cos y
ð3Þ
where q is in units of the ground state momentum of the
hydrogen atom given above. For q=0 Eq. (3) reduces to Compton’s
140
210
150
200
190
170
160
180
Fig. 3. Dependence of Klein–Nishina cross-section (cf. Eq. (2)) on scatter angle for
primary radiation of 10 (outermost curve), 100 and 1000 keV (innermost curve).
Cross-section is represented by radius vector, which is expressed in units of r02 .
equation given earlier. Tables of free atom Compton profiles J(q)
for 1rZr92 have been calculated by Biggs et al. (1975).
To give an impression of the magnitude of the Doppler
broadening effect, Fig. 4 shows the shape of the Compton peak,
derived from the tabulated values, resulting when 100 keV
photons undergo 901 scattering from aluminium.
The K shell electrons are seen to provide the major contribution to the peak at energies well away from the Compton energy.
The full-width at half-maximum height (FWHM) is approximately
1 keV, which is sufficiently wide to allow the peak shape to be
resolved using semiconductor detectors. The FWHM increases
with increasing scatter angle and photon energy. It should be
noted that Doppler broadening of the Compton peak is omitted
from several Monte Carlo programs which model photon
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0.08
0.8
Polyethylene
total
0.6
0.4
K shell
0.2
0
75
80
85
Scatter energy (keV)
90
95
Fig. 4. Energy distribution of 100 keV photons inelastically scattered through 901
from aluminium. The full line is the summed peak from all electrons. The dashed
line represents the contribution of the K electrons only.
Relative scatter intensity
Normalized intensity
1
0.06
Lucite
0.04
background
0.02
0.08
0
Relative scatter intensity
55
0.06
Be
57.5
60
62.5
Scatter photon energy (keV)
65
Fig. 6. Inelastic scatter spectrum of polyethylene and Lucite in neighbourhood of
W characteristic lines recorded using Compton spectrometer. Data taken from
Matscheko et al. (1989).
0.04
Al
0.02
0
55
57.5
60
62.5
65
Scatter photon energy (keV)
Fig. 5. Inelastic scatter spectrum of Be and Al in neighbourhood of W
characteristic lines recorded using Compton spectrometer. Data taken from
Matscheko et al. (1989).
transport. Namito et al. (1994) have described a modification to
the EGS4 code, to be discussed later (cf. Section 3.2.6), which
treats the effects of electron binding and Doppler broadening in a
self-consistent way. As expected, the integral over energy of the
Compton peak reduces to the Klein–Nishina cross-section.
2.1.3. Material characterization using the Compton profile
Although Compton profile analysis using monochromatic
radiation sources is a standard technique for determining electron
momentum distributions in the solid state, little use has so far
been made of the shape of the Compton profile in Compton
scatter imaging.
The most suggestive work to date on the feasibility of material
characterization via Compton profiles excited by X-ray emission
from a conventional X-ray source stems from the Compton
spectrometer work of Matscheko et al. (1989). Inelastic spectra
resulting from 901 scattering of a tungsten anode X-ray tube
emission from two metal samples (Be and Al) are reproduced in
Fig. 5. Each scatter spectrum is normalized to unit area between
the energy limits of 54 and 72 keV; differences in the plots arise
purely from line shape variations.
A further example of the inelastic scatter spectra reported by
Matscheko et al. (1989) from two plastic materials, polyethylene
and Lucite is reproduced in Fig. 6. It is recalled that the plots are
individually normalized to have equal unit area; hence the
significant differences evident between them originate solely in
their varying electron momentum distributions. This plot also
indicates the continuous bremsstrahlung component, estimated
as a straight line fitted to the high energy and low energy sides of
the polyethylene peak.
An energy resolution of 1.5% at 60 keV for the roomtemperature semiconductor detector CdTe has been reported
(Yadav et al., 2005); thus making material characterization based
on back-scatter Compton profile imaging a new and exciting
possibility.
The variation in attenuation coefficient, m, in inhomogeneous
objects leads to a non-uniform spatial distribution in the intensity
of the transmitted X-ray beam. As noted earlier, X-ray attenuation
is often accompanied with the production of secondary radiation.
There are thus, in principle at least, two alternative ways of
imaging variations in m: either through monitoring the intensity
of the transmitted beam or by directly observing X-ray scattering.
The interesting possibilities that Compton scatter imaging affords
will now be examined in more detail.
2.2. Comparison of scatter and transmission imaging
Although there has been considerable technical development
over the 100 years since Roentgen’s discovery of X-rays, the
principle of transmission radiography has remained unchanged
since then. This demonstrates the robustness of transmission
radiography, a formidable opponent against which X-ray scatter
imaging has to compete. The most significant points of comparison between the two in non-medical radiography are defect
contrast; measurement geometry and the system complexity
needed for tomographic imaging. These points will be expanded
in the next sections.
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2.2.1. Backscatter measurement geometry
Conventional radiography is based on the rectilinear propagation of X-rays from their point of origin, the radiation source, to
the detector where they are converted to a measurable signal.
This is an example of an ‘‘inline’’ imaging geometry, in which the
object under investigation is inevitably located between source
and detector. It is clear that such an imaging geometry, while
being intuitively simple, cannot be applied to objects of large
spatial extent; as the attenuation the X-rays suffer in penetrating
the object will prohibit measurement of the transmitted X-ray
field. This problem does not occur in the field of medical
radiography, as the human body is often fairly symmetric (head,
trunk, limbs, etc.) around its long vertical axis. Hence transaxial
sections will have approximately the same total attenuation,
irrespective of which perspective the section is viewed from.
The attenuation problem accompanying radiography of massive objects may be avoided in X-ray scatter imaging by taking
advantage of the angular cross-section shown in Fig. 3. The
angular cross-section is approximately isotropic at non-relativistic X-ray energies, well below the electron rest mass energy of
511 keV, indicating that it is certainly feasible to place the X-ray
source and the detector on the same side of the object as one
another. This possibility is without analogue in transmission
radiography. It opens the perspective for radiography of objects
that would not otherwise be accessible to X-ray imaging.
Examples of backscatter X-ray images of historical interest will
be presented in Section 5.
2.2.2. Density sensitivity of transmission radiography
A second point of comparison between X-ray scatter imaging
and transmission radiography refers to their sensitivity to small
changes in density in low-density objects. Consider for simplicity
a void (Fig. 7) or hole of diameter, w, and zero attenuation
coefficient in highly transparent material, such as a piece of fabric.
The cloth material is assumed to have an attenuation coefficient,
m, much less than that of, say, water. As illustrated in the lower
half of the figure, the hole will give rise to a change in the number,
N, of transmitted photons. (The detector is assumed to have
perfect efficiency.) Then the absolute contrast, i.e. the mean
difference in the number of photons reaching the detector along
the two ray paths shown is
jNhole Nj ¼ N0 expðmLÞwm
the difference in attenuation coefficients between the material and
air and is often small, e.g. m at 60 keV for cotton fabric is
0.1 cm 1. Hence a 1 mm diameter air porosity at this energy
produces a relative contrast of only 1%.
2.2.3. Density sensitivity of Compton radiography
Fig. 8 shows a schematic illustration of single-point Compton
radiograph in which a beam of mono-energetic photons travels
through an object of characteristic dimension, L, composed of
material with attenuation coefficient m containing a void of width,
w. The radiation signal is recorded in a detector whose angular
acceptance is limited by a mechanical collimator (e.g. pinhole
aperture). The intersection region of the primary photon beam
with the acceptance region of the detector is the sensitive point of
the system.
If the attenuation coefficient, m, is dominated by the Compton
scatter component (see Fig. 1) and the cross-section is assumed to
be isotropic (i.e. the photon energy is assumed 5mec2), the signal,
S, from the centre of the object into a detector of solid angle DO is
mL
mL
S I0 mw exp ð5Þ
exp s DO þM
2
2
where I0 is the flux of photons in the primary beam, w is the
length of beam that scatters into the detector and ms is the
attenuation coefficient of the material at the energy of the scatter
radiation. M represents the multiple scatter component (two or
more scatter events).
The energy difference between the primary and secondary
radiations will be small, following the earlier assumption that the
photon energy is 5511 keV; hence ms will be approximated in
this section by m.
The relative contrast in this case is
2jSSvoid =ðS þSvoid Þj
ð6Þ
Eq. (6) implies that the relative contrast in the absence of multiple
scatter for the arrangement of Fig. 8 is 100%, which is invariably
much greater than in transmission imaging.
2.2.4. Tomography with scattered and transmitted X-rays
The term ‘‘tomography’’ originally referred to a method of
radiological imaging in which a certain 2-D layer of an object is
rendered in focus (Gk: tomos= sharp), while non-focal layers are
ð4Þ
where N0 is the number of photons in the primary beam and it is
assumed that the hole is small enough to allow replacement of the
exponential form by its linear expansion. The relative contrast of
the hole, defined as 2jNvoid Nj=ðNvoid þNÞ, depends from Eq. (4) on
µ
997
Radiation
source
void of
height, w
Primary
beam
L
Sensitive
volume
workpiece
Object
intensity
Detector
Fig. 7. Ray path through a cavity in an otherwise homogenous medium to
illustrate calculation of contrast in transmission imaging.
Fig. 8. Schematic illustration of point-by-point arrangement for scatter imaging.
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blurred through relative movement of X-ray source and detector.
Until the development of computed tomography (CT) this was the
only method of obtaining radiographs, which showed detail of
interest free of the disturbing effects of superimposed structures
above or below the plane of interest. The phenomenon of
superimposition is fundamental to conventional radiography, in
which the degree of darkening at some point, P, on the radiograph
is related to the path integral of the linear attenuation coefficient
through the object along the line joining P to the radiation source.
Computed tomography provides a way of solving a set of
projection equations to reconstruct the spatial distribution of m in
some thin slice of material. It requires a radiation source and a
linear or planar array of detectors, which together rotate around
the object such that the reconstructed slice is generally perpendicular to the axis of rotation. In order to avoid artefacts, all the
projections must relate to the same object, which must therefore
remain stationary during the scan time. A further obvious
consequence is that the object slice must be enclosed within a
circular scan region; indeed CT works best when the object slice
itself is circular.
In contrast to CT, which is based on manipulation of projection
information, Compton scatter imaging (CSI) registers the scatter
signal from a certain volume element (voxel), defined by the
intersection volumes of the primary and scatter beams. The
geometrical limitation of CT on the shape of the object, which
requires that the object be bounded by a circle, is not present in
scatter imaging. Hence CSI allows a much broader class of objects
to be examined than CT and in many cases yields information,
which is unavailable with transmission radiographic techniques.
Compton scatter excited by radiation whose energy is low
relative to the electron rest mass energy is emitted substantially
in all directions (cf. Fig. 3). Hence the detector may be placed in
any desired position relative to the radiation source and
scattering object. The free choice of measurement geometry in
CSI leads to a possibility, excluded in CT, of monitoring the backscattered radiation from voluminous or strongly absorbing
material. In this way, images of superficial regions of a bulky
object may be obtained, as illustrated by the depth-resolved
images through a mediaeval fresco presented in Fig. 12.
For these reasons Compton scatter offers an attractive
alternative to X-ray transmission when tomographic information
about the scattering medium is required. We now describe some
typical measurement configurations for Compton scatter imaging
and their strengths and limitations.
3. Compton X-ray scatter imaging techniques
3.1. Compton imaging topologies
In the absence of reflection or refraction phenomenon (the
refractive index of all materials for diagnostic energy X-rays is a
few parts per million less than unity) a measurement of the
spatial distribution of scatter radiation can only be performed
using some kind of mechanical collimation. Several different
possibilities, using various configurations for the primary and
scatter beam geometries, have been reported over the years.
These possibilities are categorized below according to the number
of scatter voxels that are simultaneously measured.
3.1.1. Point-by-point imaging
Of historical importance is the point-by-point imaging scheme
used originally by Lale (1959) and illustrated in Fig. 8. Absorbing
apertures collimate radiation from the source to form a primary
(‘‘pencil’’) beam, which approximates a 1-D line travelling in the
Z direction. A single detector unit (e.g. scintillator/photomultiplier
combination) is arranged to record scatter radiation through a
collimator, which focusses down to a point in the XZ plane. This
system measures sequentially the scatter from a series of points
lying along the Z axis; hence provision must be made to move the
object in the XZ plane to derive a 2-D raster slice image. In
addition to the complicated mechanics the system is dose
inefficient, as material along the line of the primary beam
receives radiation dose irrespective of whether its scatter is
recorded or not.
It is instructive to consider in the point-by-point imaging case
the increase in measurement time, T, which accompanies an
increase of the spatial resolution with which an object is to be
imaged (e.g. from 2 2 2 mm3 to 1 1 1 mm3). We assume
that the object is partitioned into cubic voxels of edge length, w,
and that the number of scatter photons measured from each voxel
is constant to maintain a constant signal-to-noise ratio.
First the cross-sectional area of the primary beam and thus the
solid angle subtended by the beam at the source varies as w2.
Moreover the scatter signal depends from Eq. (5) on the length of
the beam (in the propagation direction) viewed by the detector
through the secondary collimator, which varies as w1. Similarly
the number of voxels that have to be sequentially addressed in a
fixed volume of the object varies as w 3. Finally an increase in the
spatial resolution of the scatter collimator can generally be
obtained only by reducing the solid angle it subtends at the
beam. These considerations imply that the measurement time T
varies approximately as w 7 if the source radiance and the
contrast discrimination remain constant. It should be noted that a
similar relationship holds for computed tomography (CT). The
only practicable ways of reducing measurement time are to
increase the radiance of the source, the area of the detector, or
both. The study of high power X-ray sources and cheap, large area
detectors, is almost obligatory to those involved in the development and exploitation of X-ray scatter imaging techniques. In the
next section we consider ways of reducing the measurement time
by effectively increasing the detector area.
3.1.2. Line-by-line imaging
It was shown above that the point-by-point imaging scheme
makes poor use of the available scatter and therefore results in
lengthy measurement procedures. As a further development, the
scatter from each voxel irradiated by the primary beam may be
measured simultaneously using a linear detector array, as
developed for CT and other applications. Typical collimating
elements for the scatter radiation are either a series of plane
lamella, such that each element of the detector array ‘‘sees’’
through the collimator its own region of primary beam (corresponding to the multi-channel collimator of radionuclide imaging) or a slit, whose extension in a plane perpendicular to the
primary beam direction is much greater than in the parallel
direction. The efficiency and spatial resolution characteristics of
multi-hole and pinhole collimators in nuclear medicine have been
discussed by Barrett and Swindell (1981). Owing to the usual
obliquity effect the pinhole is inferior to the multi-hole if it
subtends a large angular range (greater than, say, 451) at the
source of scatter. Otherwise the pinhole offers several advantages
in practice. These include (a) a variable choice of magnification;
(b) simplicity of manufacture for high resolution (sub-millimetre)
imaging; (c) reduction of secondary scatter from the collimator at
high photon energies through judicious shaping of the sides of the
pinhole; (d) the possibility of adapting the collimation scheme to
reduce attenuation effects (see below); and (e) incorporation of a
multiple scatter grid in the collimator between the pinhole and
the detector. The ComScan device which will be described in
Section 4 incorporates for these reasons a slit collimator.
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object
pinhole
radiation source
999
The method has in practice several drawbacks. The need for
monochromatic primary radiation restricts the method to radionuclide sources (137Cs at 660 keV is frequently used) and this
leads to long measurement times. The limited energy resolution
of practical detectors (commonly Germanium) and incomplete
energy conversion within the detector, owing to photons leaving
the detector as Compton scatter or fluorescence radiation, both
complicate the relationship between the measured energy
spectrum and the object density profile. The detector must accept
scatter over the whole field of view that is to be imaged and hence
multiple scatter is significant. The images derived from this
technique display considerable noise and have a spatial resolution
around 1 cm. For these reasons there has been little activity in the
field of energy-coded scatter imaging.
2-D detector
3.2. Densitometry with scattered X-rays
Fig. 9. Schematic illustration of plane-by-plane arrangement for scatter imaging.
3.1.3. Plane-by-plane imaging
Owing to the ready availability of commercial 2-D detectors
(e.g. X-ray film, image intensifier/TV chain or the Anger-type
gamma camera used in radionuclide imaging) several workers
have presented scatter images derived using simple pinhole
collimation optics as depicted in Fig. 9.
Harding (1985) presented sectional images of various aluminium castings, whereas Guzzardi and Licitra (1988) examined the
utility of Compton scatter imaging for monitoring lung function
and disease. Common to the sets of images presented by both
groups is the attenuation effect, which shows itself as an intensity
gradient on which image detail is superimposed. The situation can
be partially remedied by arranging for the object to be illuminated
from opposite sides, which reduces primary attenuation artefacts,
and using two detector systems that similarly compensates to
some extent for attenuation of the scattered radiation.
The plane-by-plane measurement configuration is very easy to
set up and readily provides moving images (e.g. of the heart)
when a dynamic detector is used. However, it is not in current
favour for the following reasons. In spite of the large detector area,
it is no more sensitive than the line-by-line technique, as the
increase in the number of picture elements it offers exactly offset
by the lower solid angle of the pinhole compared to the slit
collimator. Further there is no possibility of discriminating with
mechanical collimation against multiple scatter radiation. Finally
areal detectors are, at least at present, more expensive than their
1-D counterparts, although this situation may change when large
area dynamic detectors based on amorphous Si arrays become
commercially available (Antonuk et al., 1994).
3.1.4. Energy-coded scatter imaging
The final method of scatter imaging that will be described here
relies on the Compton relationship (Eq. (1)) between the angle of
scatter, y, and the energy, E1, to which the primary radiation of
energy E0 is shifted. If an object is irradiated by a well-collimated
beam of monochromatic primary radiation, it is possible in
principle to determine the point of origin of scattered photons
of energy E1, since these must travel along a path to the detector
which makes an angle of y to the primary beam. If a small energyresolving detector is arranged to monitor the emergent scatter, its
energy spectrum is thus related to the density profile of material
in the path of the primary beam. Position information in the
object is encoded, through the scatter angle, in the energy
spectrum of the scatter. This method and preliminary results
were first presented by Farmer and Collins (1974).
The ultimate goal of radiography is not just to give a
qualitative image of the morphology of the object in which
structures may be visually recognized, but to provide the
quantitative spatial distribution of some well-known physical
parameter such as linear attenuation coefficient, m, density, r, etc.
The multiple scatter component (Eq. (5)) has so far been ignored,
representing the effect of photons liberated along the primary
beam ray path that are multiply scattered in the rest of the object
and emerge from it along the path of the secondary radiation. It is
clear that, whereas spatial definition of the sensitive volume
element is straightforward, the relationship between measured
signal and the electron density is complicated.
We now examine various techniques for determining the
attenuation factors.
3.2.1. Numerical corrections for attenuation effects
As discussed previously, at the photon energies typically used
for X-radiography, Compton scattering is the predominant
contribution to the total attenuation coefficient. In this case both
S and m in Eq. (5) depend only on the electron density and the
following simple attenuation correction suggests procedure itself.
Consider one corner of a 2-D slice through an object. Let the slice
be segmented into voxels (i, j) which are small enough that m is
practically constant in each of them. We assume that the set of
scatter signals S(i, j) has been measured. Then the electron
densities re(i, j =1) of the top row of voxels (nearest the source
and detector) can be determined from Eq. (5) since they represent
the surface of the object and are thus not affected by attenuation.
Knowing the scatter coefficient for this row of voxels allows the
primary and secondary attenuation they induce in the scatter
signals of the second row to be calculated. This procedure can be
repeated for as long as is judged worthwhile. In practice there are
two limitations. First the statistical uncertainty in the scatter
signal leads to an error in estimation of the attenuation. This error
exceeds the pure photon noise error source when mL41, where
L is the combined length in the object of the primary and scatter
radiations. Second there is a systematic error which arises
through neglect of the multiple scatter term, M. The effect of
this is to overestimate the attenuation factors, leading to an
exponential increase in the derived value of electron density.
Naturally some approximate information on the dependence of
multiple scatter on the object geometry can be incorporated into
the correction procedure, this information being derived from
measurement data, theoretical considerations or the results of
Monte-Carlo simulations (see Section 3.2.6). Nevertheless, as a
rule of thumb, quantitative data from scatter measurements
obtained by numerical correction procedures should be seriously
questioned if the mLo1 condition is not satisfied.
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3.2.2. Dual energy determination of attenuation factors
Owing to the limitation of numerical correction procedures to
small objects (mLo1) or object regions close to the surface,
several investigators have examined the potential of ‘‘dual
energy’’ techniques for attenuation correction (Huddleston and
Weaver, 1983; Harding and Tischler, 1986). ‘‘Dual energy’’ refers
to the measurement of two scatter signals under identical
geometric conditions using primary beams of different (mean)
energy El and Eh, where the subscripts l and h refer to ‘‘low’’ and
‘‘high’’ energy, respectively. The two energies may be provided by
a single radionuclide (e.g. gadolinium-153 with Eg = 41 and
100 keV) combined with energy-sensitive detection or, from a
bremsstrahlung source, by altering the X-ray tube potential, or by
the use of source or detector filtering schemes. The dual energy
technique is based on the physical fact that for photon energies in
the range 30 keVrEr150 keV the linear attenuation coefficient
of a material with 1rZ r20 may be parameterized to an accuracy
of 0.5% using a linear combination of only two terms, i.e.
mðEÞ ¼ aC fC ðEÞ þ ap fp ðEÞ
ð7Þ
The functions fC and fp containing the energy dependence of m can
be chosen according to convenience: they are frequently assumed
to refer to Compton scattering and photoelectric absorption. In
this case the material-specific coefficients aC and ap are identified
with the local electron density and mean atomic number (raised
to some power), respectively. Provided that multiple scatter
(Eq. (5)) is known or negligible and that the ratios of El0 /El and
Eh0 /Eh (the primes refer as above to Compton shifted energies) are
approximately equal, Eqs. (5) and (7) indicate that there are three
unknowns, i.e. the electron density, the Compton and the
photoelectric components of the total attenuation. The two
scatter signals Sl and Sh obtained at low and high mean photon
energy provide sufficient information to allow two of these
unknowns to be measured (Harding and Tischler, 1986).
3.2.3. Multiple scatter
Whenever the optical thickness m, L of an object is comparable
to or greater than unity (m is the linear attenuation coefficient and
L a typical width characterizing the object) the probability is
significant that there is a multiple scatter contamination of the
single scatter signal. Eq. (1) gives the Compton relationship for
single scatter between the angle of scatter, y, and the energies E0
and E1 of the primary and scatter radiation. If a photon is deflected
in n steps through a total scatter angle of y, it is easy to show that
its maximum energy is
En ðyÞ ¼ E0
E0 n ½1 cosfy=ng
1þ
me c2
ð8Þ
The symbols in Eq. (8) have their usual meanings. It indicates that
multiple scatter can have significantly higher energy than single
scatter deflected through the same angle: for the case of large n,
the cosine term reduces to unity and En(y)EE0. It is often
impossible to distinguish on the basis of photon energy between
(Doppler-broadened) single scatter and multiple scatter. Hence an
account is now given of the degrading effect of multiple scatter in
Compton scatter imaging and some of the ways it may be
corrected for or otherwise taken into account.
Consider a body of uniform density, r, which yields a total
signal made up of a single scatter S1 and multiple scatter Sm
contribution. In the presence of a small defect of density r0 the
single scatter component changes but the multiple scatter
remains unchanged. The relative contrast, Cm, of the defect in
the presence of multiple scatter radiation is related to its
undegraded contrast, C, by
Sm
Cm ¼ C 1
St
ð9Þ
According to Eq. (9) the apparent contrast in the presence of
multiple scatter is reduced relative to the true object contrast by
the factor S1/St, often abbreviated to STSR, the single-to-total
scatter ratio. It is therefore very important to be able to quantify
and hence maximize the STSR of a certain measurement if reliable
statements of the true object contrast are to be made and the
measurement time is to be reduced.
There is a vast body of literature on inelastic X-ray scatter
imaging and analysis. The interesting question arises as to the
how the degree of multiple scatter contamination in the total
scatter signal is generally determined. Perusal of the many articles
available reveals that the overriding tendency has been to ignore
multiple scatter in the hope that its effects will not be too serious.
Neglect of multiple scatter is only justified in measurement
geometries for which the probability is small that the primary
radiation will be scattered at all in the object, i.e. mLo1. When
this is not valid, recourse must be made to one of the following
approaches.
3.2.4. Multiple scatter reduction
The problem of X-ray scatter contamination of transmission
radiographs is well-known. The simplest way to reduce scatter is
to use the fact that, whereas the transmitted X-rays originate
from a point-like focus, the scattered radiation has its effective
origin inside the patient. Hence by arranging for a grid,
comprising many narrow channels which converge at the tube
focus, to be placed between patient and detector, the angular
acceptance of the detector to X-ray scatter can be radically
reduced. In exactly the same way, use can be made of the fact in
X-ray scatter imaging that the single scatter originates in the
primary beam. Hence a grid composed of many lamellae which
intersect at the primary beam is an effective means of increasing
the STSR in a line-by-line imaging system (cf. Section 3.1.1). As
confirmed by experiment and in accord with expectation, the
degree of reduction is simply proportional to the number of
lamella, or inversely proportional to the angular acceptance of the
detector. It is evidently advantageous to arrange for the primary
beam to have low cross-sectional dimensions so that the angular
acceptance of the multiple scatter grid can be made small without
absorbing the desired single scatter radiation.
One limitation of mechanical collimation as a means of
reducing the multiple-to-single scatter ratio is that it requires
fixed beam-detector geometry. This is a problem for those
systems (such as ComScan), which scan the object with a beam
deflection mechanism. Hence we now consider ways to numerically correct the total scatter signal for the effects of multiple
scatter.
3.2.5. Multiple scatter calculation
A very general (and usually correspondingly complicated)
approach to scatter problems is formal transport theory based on
the Boltzmann equation. Originally developed in connection with
the kinetic theory of gases, the Boltzmann equation describes the
temporal development of a phase space distribution function
involving the momentum and position co-ordinates of a photon.
A discussion of the application of the Boltzmann equation to
radiological problems is given by Barrett and Swindell (1981). The
equation is tractable only when major simplifying assumptions
can be made, e.g. that each photon is scattered many times with
negligible energy loss. For this reason calculations based on the
Boltzmann equation of multiple scattering in the Compton regime
covered by this review have not been attempted.
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3.2.6. Multiple scatter modelling
The most frequently used approach to derive numerical values
of the STSR is through Monte-Carlo modelling. In this approach
the path of each photon through the (generally inhomogenous)
medium is simulated using distribution functions based on
random number generators to determine such collisional parameters as the type of interaction (Compton, pair production,
photoeffect, etc.), the polar and azimuthal angle of the scattered
particle (photon or Compton electron) and the track length to the
next interaction. Of the several Monte Carlo codes that are
available one of the most widely used is EGS (electron gamma-ray
shower). It is not possible to go into any details of the code, which
is described in numerous publications (Nelson et al., 1985).
4. Description of ComScan system
The preceding sections have considered the physical principles
on which Compton scatter imaging (CSI) is based. The next
sections illustrate these principles and the development from
them of a commercial CSI system—the ComScan marketed by
Yxlon International GmbH—which is illustrated in cross-section
in Fig. 10. Finally, images derived from some applications of
historical interest are presented.
4.1. General considerations
Since radiography is a mature technique, which is widely
applied in non-destructive testing (NDT) it was decided to exploit
those features of X-ray Compton scattering imaging that have no
analogue in transmission imaging. The first of these is the
availability of scatter radiation in all directions (cf. Fig. 3). Since
transmission radiography inevitably locates the object between
the radiation source and sensor it was decided to develop a backscatter technique, which would be able to image superficial
regions of massive objects, whose sheer bulk precludes the
possibility of transmission imaging.
Structure under
investigation
Collimation
slit
Detector
array
x-ray tube
Fig. 10. Cross-section through ComScan measurement head perpendicular to
primary beam deflection direction showing 22 element detector array, primary
beam channel, deflector unit and scatter collimation slit.
1001
Secondly, the decision was made to perform 3-D localization of
structures in the scattering medium, rather than just measuring
the total back-scatter flux, as implemented by other workers
(Towe and Jacobs, 1981). The use of a fine primary beam (crosssectional dimensions are 0.5 mm 0.5 mm) reduces the uncertainty in the position of a scatter voxel to 1-D; hence it is only
necessary to image the scatter by means of a collimation element
(cf. Section 3.1.1) onto a detector array with spatial resolution to
uniquely relate the signal from a certain detector element with
the 3-D location of the scatter voxel.
Finally the preferred realisation of ComScan was as a compact
measuring head, which could be affixed to a regular industrial
X-ray tube. The head (Fig. 10) was designed to contain the major
components of the scatter imaging device, i.e. primary beam
deflector, detector array and collimator for the scatter radiation. A
more detailed discussion of these components will now be given.
4.1.1. Radiation source
The radiance (photons per unit source area emitted into unit
solid angle per second) of a commercial X-ray tube is several
orders of magnitude greater than that of practical radionuclides.
This increased photon flux directly translates into reduced
measurement time. The ComScan system is generally supplied
with an x-tube having a maximum potential of 160 kV. The mean
photon energy when the tube is operated at this voltage depends
on the degree of filtering but is typically 60 keV. The photon
radiance of a conventional (reflection geometry) W anode tube
operated at 160 kV is approximately 10 13 photons s 1 mA 1
sr 1.
4.1.2. Primary beam collimation
The primary beam diaphragm has to provide a finely
collimated beam which can be linearly scanned over a distance
of 60 mm at the object with a scan frequency of up to 10 lines
per second. Moreover the scan mechanism must be compact
enough to fit into the ComScan head and provide high absorption
even for 160 kV radiation. The traditional solution to this
problem—the Nipkow disc—proved too bulky and instead an
arrangement of two coaxial rotating cylinders, each equipped
with diametrically placed helical slits, was developed. The flux of
photons in the primary beam (cross-sectional dimensions of
0.5 mm 0.5 mm) is approximately 2 10 9 s 1.
4.1.3. Detector array
In order to maximize the detector solid angle while allowing
for the linear scan movement of the primary beam, each detector
element is a scintillator strip of 60 mm length (parallel to the
deflection direction) by 2 mm depth, to provide high stopping
power, by 1 mm effective thickness in the vertical direction.
Owing to the high refractive index and superb optical quality with
which inorganic scintillators can be grown, the scintillator crystal
acts effectively as its own light guide, transporting scintillation
light to the two small end faces. Each scintillator crystal is glued
at these faces to plastic fibre light-guides, which transfer the
scintillation light to a photomultiplier. There are two arrays of
detector elements, each containing 11 elements, arranged on
either side of the primary beam as shown in Fig. 10. The vertical
separation of adjacent elements is 1.5 mm. The two arrays are
displaced by one half of this value from one another to provide
detector signals sampled at double the spatial frequency. It is then
possible to process these signals to increase the spatial resolution
and reduce aliasing effects.
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4.1.4. Scatter collimation
It is a design specification of the ComScan to image structures
to a maximum depth of 50 mm with a variable spatial resolution
of at best 0.5 mm in all three dimensions. To fulfil these
requirements ComScan is equipped with a set of two scatter
collimators, providing a magnification, which varies from 3 (5 mm
object depth) to 1/3 (50 mm object depth). The scatter collimators
are wedge-shaped and provide a slit opening of 50 mm length
having a width dependent on the magnification factor. They are
manufactured from a tungsten alloy material and it is a fairly easy
job to change from one set of collimators to the next when the
magnification or field depth is to be changed.
4.2. Data acquisition and processing
The X-ray pulses recorded by the photomulipliers are
amplified, shaped and counted in 22 memory buffers for
0.25 mm increments in the position of the primary beam as it
sweeps out a linear path. The acquisition time is typically 2 ms per
voxel (22 voxels are measured simultaneously) corresponding to
0.25 s for a 2-D slice and approx. 2 min for a 3-D data set
comprising 512 such slices. As a standard procedure the data are
rearranged to provide 22 slices lying in planes perpendicular to
the propagation direction of the primary beam and with
increasing depth into the structure being investigated, each slice
image having 512 256 pixels. Various display and data processing options can be applied to the data. The former include a
variable look-up table (LUT) to relate the number of photons
measured from a voxel to a grey scale or a pseudo-colour scale.
Among the data processing options are image smoothing and
enhancement schemes and some quantitative algorithms for
determining statistical properties of the image (lengths, areas,
mean photon number, standard deviation, etc.).
All of the images presented in this review employ standard
data resorting yielding object slices, which are perpendicular to
the plane of the drawing in Fig. 10.
5. Historical applications of ComScan
It is not the intention here to give an overview of all the
applications which ComScan has found in the field of nondestructive testing (NDT): these are published in specialist
ComScan reports obtainable from Yxlon Industrial X-ray,
D-22315 Hamburg. Instead we highlight the significance of an
interdisciplinary approach by presenting some applications of
ComScan in the fields of archaeology, art and culture. Some of this
material has been presented elsewhere (Niemann, 1993).
The aim of this section is to exemplify the application of
scientific tools to historical research. However, strictly speaking
pre-modern historiography, pre-historic history and archaeology
are separate academic disciplines and therefore have their own
individual methodologies.
For many years, written sources, such as books, letters or other
paperwork, functioned as cultural history’s sole source. Recently,
however, artefacts and their symbolic displays have been
increasingly emphasized. Paintings and other craftworks are not
seen merely as representing an art style or epoch but are
examined in their historical context. This includes a growing
interest in a craftwork’s symbolic pattern as well as the dimension
of how it originated in and impacted on political formations.
In order to explore the historical significance of a work of art,
questions are being raised such as: who commissioned it; why
was it commissioned; in what context was it employed; or where
was the object positioned? This perspective aims to give a more
precise idea of the use and interplay of visual signs and human
interaction. As a result of this shift, current research considers
early modern, medieval or ancient meeting places with a focus on
exchanged gifts, banquets, architectural arrangements of court
halls; as well as burials and donated grave offerings, to list just a
few examples (Burke, 2001; Duindam, 2003; Roeck, 2004).
5.2. History and natural sciences
However, historical research has to rely on other academic
disciplines when seeking a comprehensive understanding of
historical contexts. Though written sources often give an insight
into the origins and history of artefacts, background information
cannot always be retrieved by consulting written material stored
in archives. Especially when dealing with pre-literate epochs,
natural sciences such as bio-chemistry and physics help to
explore and reveal hidden aspects (Eggert, 2005). When applied
to art or cultural artefacts, analytical techniques developed in the
natural sciences can determine the materials and technologies of
fabrication as well as charting chronological modifications. In this
sense, the natural sciences provide tools to conservators and
academic scholars, enabling them not only to explore history, but
also to restore it.
5.3. Applications and interpretations
The Compton back-scatter technique (ComScan) is an example
of a tool from physical sciences that has been applied to historical
artefacts. Fig. 11 illustrates a scene from a fresco on the second
5.1. Scientific approaches to history
Modern historiography, influenced by an increasing awareness
of cultural changes in today’s society, focuses on various forms of
interaction as a key to understanding social values, norms and
identities. It tries to uncover how pre-modern social order, rank
und distinction was manifested and renewed in interaction by not
only examining how distinguished individuals (e.g. politicians or
sovereigns) demonstrated their power, but also by looking at the
everyday life of different communities. To a great extent social
boundaries and cultural conflicts between groups result from
conflicting value systems. Categories of self-perception and how
they are visually expressed are therefore important subject
matters historians need to deal with, allowing modern societies
and the various issues they are facing to be understood in a more
profound way (Muir, 2005; Burke, 2004; Stollberg-Rilinger, 2004).
Fig. 11. Application of ComScan to restoration of damaged fresco.
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floor (Landgrafenzimmer) of the Wartburg castle in Eisenach,
Germany. The Wartburg, included in the World Heritage List since
1999 and famous for visitors such as Luther and Goethe
(Schuchardt, 1996), not only counts as one of Germany’s most
picturesque castles but was also a central lieu de mémoire in
Germany’s 19th Century strive for national identity (Etienne,
2001). In this period, its owner, Grand Duke Carl Alexander von
Sachsen-Weimar-Eisenach (1818–1901) undertook extensive
restorations and reconstructions (Schall, 1993) in the
Landgrafenzimmer and other parts of the building. The duke
explained his motives with the following remark (in translation):
‘‘My wish is to refashion the Wartburg as a museum of our
dynasty, of our country, and moreover of all of Germany’’.
However, these modifications have to be seen in the light of
their time (Nipperdey, 1976). This is notably true for the frescos
by Moritz von Schwind (1804–1871), which were influenced by
contemporary—and, from today’s perspective, inaccurate-conceptions of mediaeval times. After having been neglected for
many centuries, the castle now was regarded as a medieval
monument documenting the roots of a German identity. Hence,
von Schwind was commissioned to revive the middle ages
according to an idealised, contemporary picture of the past. But
the frescos he created, like those of the legends of the castles’
origins (1854), which can be found in the Landgrafenzimmer,
resulted from nothing more (nor less) than an invention of
tradition (Hobsbawm, 2007).
Modern historiography tries to uncover these inventions, not
primarily to expose them but in order to understand concepts of
Fig. 12. (Cf. Fig. 11) False-colour scatter images at depths into wall of 2, 4 and
8 mm from left to right showing depth dependence of texture.
1003
identity and mechanisms by which social groups were and are
formed (Etienne, 2001).
As evident in Fig. 11 some parts of the fresco have been
damaged by moisture. To the right of the figure can be seen the
ComScan measuring head.
Fig. 12 shows section images at three depths into the wall.
The first section on the left of the figure is within the pigment
layer of the fresco. The other two images show the transition from
a fine-grain plaster (centre) to a coarse-grain cement (right). In
this way it is easy to determine which kind of technique the
craftsmen originally used for the fresco. Images similar to those
shown in Fig. 12 helped to locate a large stone and a surrounding
fissure in the cement through which moisture was attacking the
fresco. Applied to this fresco, ComScan can assist in protecting
cultural heritage. Besides contributing to ‘‘restoring history’’,
further research will examine what materials and techniques
were used in the attempt to create a supposedly medieval
surrounding, this attempt itself providing valuable information
on the relationship of Grand Duke Alexander and von Schwind to
the mediaeval society they sought to portray.
However, applying this method to layers hidden behind the
fresco seems even more profitable. Behind the fresco we
encounter periods in history with hardly any written sources.
Here finally, this method enables sparse source material to be
supplemented; allowing thus, to raise and approach questions as
to how architecture and the symbolic arrangements of a room
defined social interaction and order.
Fig. 13 shows a sagittal section through a mummified body from
Central Africa. One interesting feature of ComScan is its ability to
image directly sagittal sections (parallel to the long axis of the body)
in contrast to computed tomography, which yields only transaxial
sections. In order to image the body at such high spatial resolution
several hundred transaxial sections would have to be measured
which is both exceedingly costly and time-consuming. As part of the
preservation process the body is wrapped in layers of cloth. These
layers reveal themselves as the linear and folded structures visible in
the image. The shapes and thicknesses of these layers of cloth allow
conclusions to be drawn on the geographical origin and social ranking
of the mummified body.
Fig. 14 is a false-colour image of a mediaeval bronze clasp
(approx. 5 cm diameter) that was typically used to bind together
the two corners of a cloak around the shoulders. The bronze has
corroded at the surface, thus preserving bits of cloth in the
interior of the clasp. An X-ray transmission study was performed
at high energy (120 kV) in order to penetrate the bronze: no
internal structure of the textile in the clasp could be seen. With
ComScan useful details of the internal structure of the cloth
became apparent.
Both examples illustrated in Figs. 13 and 14 supply information
for further research when examining social order in pre-modern
times. ComScan can contribute to specify the geographical
Fig. 13. Sagittal section image through Central African mummified body, showing several layers of textiles.
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References
Fig. 14. False-colour CSI image of one section through a mediaeval bronze
clasp—Radkopfnadel—(Schleswig, 2007).
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In a broader approach this information would enable historians to identify various medieval strategies of demonstrating
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weaponry or horses, an analysed object turns out to be a grave
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on early medieval times has suggested that these gifts or
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death but also in the event of burial (Brather, 2004; Haerke, 2001).
6. Conclusions
Compton scattered X-rays have been used as the basis for a
radiological imaging modality having the following, unique
characteristics: high contrast resolution, sufficient to resolve the
weaving pattern in ancient fabric (Fig. 13); direct 3-D spatial
resolution of structures inaccessible to computed tomography
(Fig. 12); and a reflection geometry (Fig. 10) allowing images to be
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X-ray source and detector unit.
The use of room-temperature spectroscopic detectors to
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The ComScan back-scatter imaging device described here has
been applied as a tool for interdisciplinary historical research. Its
results exemplify ways in which scientific information gleaned
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this work will encourage other natural scientists and historians, in
particular, to engage in interdisciplinary research to the benefit of
their respective communities.
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The co-operation of the Wartburg Stiftung for Figs. 11 and 12
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